People keep talking about Ronaldo but Ronaldo isn't the problem IMO.
He might not be the problem but he adds to it. You're trying to play a 4-2-3-1 with 4 forwards in the side. Trying to play out and out forwards in the wide positions of a 4-2-3-1 is a massive ask and even with harder working players and a stronger base behind them, you're going to get exposed. Add Ronaldo and the fact that he doesn't run into that and you're going to struggle.
Look at our first goal yesterday. It's a direct result of Utd trying to press from the front with a CF that doesn't move and a two man midfield. Fernandes ends up leaving his number 10 position to press our keeper, Greenwood then follows him by attempting to press VVD and without that extra midfielder Wan Bisakka's then trying to press Robertson - 3 easy passes and Jota's got the ball midway in your half with half your side taken out of the game.
Now you could argue that it was crazy of Utd trying to press in those circumstances but having a CF that would run would have prevented what happened. Fernandes isn't vacating his position to start the press and as a result neither are Greenwood and AWB.
This Utd squad is confused. You're playing 4-2-3-1 because it's the only way you can shoehorn everybody into the system but it doesn't really work.